There are three major OBJECTIVES in this application, each of which addresses pathways of hepatic lipid metabolism implicated in human disease. The first objective is to advance understanding of cell specific (hepatocyte versus stellate cell) lipid droplet composition and turnover in modulating hepatic fibrogenesis. The second objective is to understand how genetic modifiers of hepatic VLDL assembly regulate lipid droplet formation and fibrogenic signaling. The third objective is to understand how genetic modifiers of VLDL assembly and of fatty acid uptake also modify canalicular cholesterol secretion and gallstone susceptibility. The BACKGROUND to the current proposal is based on key published and preliminary findings implicating lipid droplet formation and fibrogenic signaling and also data linking pathways of VLDL assembly to gallstone susceptibility. The AIMS of this proposal will test the following HYPOTHESES. Based on observations that germline L-Fabp deletion attenuates lipid droplet formation in both hepatocytes and stellate cells, AIM (1) will test the hypothesis that L-Fabp modulates lipid droplet formation and fibrogenesis associated with steatosis via cell specific pathways. Based on observations that germline L-Fabp deletion attenuates hepatic lipid droplet formation and steatosis in the setting of liver-specific Mttp deletion and also attenuates hepatic fibrosis associated with high trans fat fructose feeding, AIM (2) will test the hypothesis that VLDL assembly pathways modulate lipid droplet formation, FA trafficking and fibrogenic signaling. AIM 2 will also examine iPS derived human hepatocyte like cells. Based on observations that liver-specific Mttp deletion attenuates gallstone formation in both the wild type and L-Fabp knockout background, AIM (3) will test the hypothesis that Mttp modifies canalicular cholesterol flux in addition to VLDL secretion. Based on other observations that CD36 deletion attenuates gallstone formation in both wild type and L-Fabp knockout background, AIM (3) will also test the hypothesis that CD36 modifies canalicular cholesterol secretion. Taken together, these studies address key issues in understanding the pathways that mediate progression of benign hepatic steatosis to fibrogenic liver injury, including the lipid mediators associated with lipid droplets. These studies also address key pathways in the genetic regulation of canalicular lipid transport and cholesterol gallstone susceptibility.